The present invention relates to a method for preparation of sintered permanent magnets superior in magnetic properties.
Japanese Patent Publication Hei 7-78269 (Japanese patent application Sho58-94876, the patent families include U.S. Pat. Nos. 4,770,723; 4,792,368; 4,840,684; 5,096,512; 5,183,516; 5,194,098; 5,466,308; 5,645,651) discloses (a) RFeB compounds for permanent magnet containing R (at least one kind of rare earth elements including Y), Fe and B as the essential components, and having a tetragonal crystal structure with the lattice constant C0 of about 12 xc3x85, and each crystal grain being separated by a non-magnetic phase; or (b) RFeBA compounds for permanent magnet containing R, Fe, B and element A (Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sb, Ge, Sn, Zr, Hf, Cu, S, C, Ca, Mg, Si, O or P) as the essential components, having a tetragonal crystal structure with the lattice constant C0 of about 12 xc3x85, and each crystal grain being separated by a non-magnetic phase. It is mentioned that the permanent magnet exhibits specifically superior properties when the above-mentioned tetragonal compounds have appropriate crystal grain sizes, the compounds constitute the major phase, and microstructures containing R-rich non-magnetic phases are formed.
According to Example 2 of the Japanese patent publication Hei 7-78269, for example, an alloy of 8 atomic % B, 15 atomic % Nd and the balance Fe was pulverized to obtain alloy powder having average particle size of 3 xcexcm. The powder was compacted under 2t/cm2 pressure in a magnetic field of 10 kOe, and then sintered at 1100xc2x0 C. for 1 hour in Ar of 2xc3x9710xe2x88x921 Torr. The permanent magnet obtained showed Br=12.1 kG, Hc=9.3 kOe, and (BH)max=34 MGOe. It is disclosed that the major phase (magnetic phase) of the sintered compact was a tetragonal compound containing simultaneously Fe, B and Nd, having lattice constants of A0=8.80 xc3x85 and C0=12.23 xc3x85, occupying 90.5 volume % of the sintered compact, and that among the non-magnetic phase separating the tetragonal compounds and forming grain boundaries of the major phase, a non-magnetic compound phase containing more than 80% of R occupied 4 volume % and the remainder was virtually oxides and pores.
Though the magnet showed excellent magnetic properties, latent abilities of the RFeB or RFeBA tetragonal compounds have not been exhibited fully. This may be reasoned on insufficient orientation of the tetragonal compounds toward the major axis direction, because the phase containing a large amount of R constituting the non-magnetic phases for separating mutually the major phases composed of the tetragonal compounds is amorphous.
The object of the invention is to provide a method for preparation of sintered permanent magnets having excellent magnetic properties by exhibiting fully latent abilities of the mother alloy for permanent magnet having a rare earth element, Fe and B as the essential components.
The method for preparation of sintered permanent magnets according to the present invention comprises the steps of: mixing fully fine powder of a crystalline mother alloy for permanent magnet containing a rare-earth element, Fe and B as the essential components with fine powder of zinc oxide, compaction molding the resulted mixture in the presence of a magnetic field, sintering the compacted mixture in vacuum to cause generation of oxygen and metallic zinc by thermal decomposition of the zinc oxide; segregation of a part of metallic component in the mother alloy at the boundary and inside of the mother alloy crystal; formation of amorphous metallic oxide by forced oxidation of the segregated metal with the generated oxygen; crystallization of the amorphous metallic oxide; formation of an epitaxial junction between the crystallized metallic oxide and the mother alloy crystal; and evaporation of the metallic zinc into the vacuum, and quenching the sintered compact.